Seasonality in phosphorus release rates from the sediments of a hypereutrophic lake under a matrix of pH and redox conditions

Phosphorus release rates were measured on intact sediment cores collected f rom the major depositional basin of Onondaga Lake, a dimictic, calcareous, hypereutrophic system in Syracuse, N.Y., U.S.A. Release experiments were co nducted under a matrix of redox and pH conditions to investigate the import ance of Ca- and Fe-related physicochemistry on sediment cores collected sea sonally, i.e., during the spring, summer, fall, and winter periods. Strong seasonal variation in P release was observed, with rates ranging from simil ar to 3 to 38 mg P.m(-2).day(-1). This variation is attributed to changes i n redox status and P concentration gradients at the sediment-water interfac e. An oxidized microlayer at the sediment-water interface partially inhibit s sediment P release under isothermal, well-mixed conditions in the spring and fall. Phosphorus trapped in the oxic microlayer (sorption) is freed whe n the microlayer is chemically reduced at the onset of anoxia and high P re lease rates are observed. The oxidized microlayer serves to regulate season ality in rates of sediment P release but does not influence long-term sedim ent-water exchange. It is proposed that the long-term P release process is best represented by a time-weighted annual average rate, calculated here to be similar to 10 mg P.m(-2).day(-1).